Technical Field
The present invention relates to a turbulence element and a rotor for a screening apparatus. The turbulence element and the rotor of the present invention are particularly suitable for use in connection with screening apparatuses of the pulp and paper industry.
Background Art
The screening apparatus used nowadays in the pulp and paper industry is almost without exception a pressurized screening device i.e. a so-called pressure screen into which the pulp to be screened is introduced in a pressurized state. The most popular pressure screens comprise a stationary screen cylinder and a rotating rotor in cooperation therewith. The purpose of the screen cylinder is to divide the fresh pulp or the fiber suspension entering into the screening zone where the rotor rotates into an acceptable fiber fraction called the accepts, and a rejectable fiber fraction called the rejects. The screen cylinder as well as, naturally, the rotor are located inside a screen housing having ducts for both the fresh fiber suspension, the accepts, and the rejects. Normally, the inlet duct or inlet for the fiber suspension is at one end of the screen housing, whereby the rejects outlet is at the opposite end of the housing. The accepts outlet is in communication with the accepts zone which is positioned at the opposite side of the screen cylinder in relation to the screening zone. The purpose of the rotor is to create turbulence, and positive and negative pressure pulses in the fiber suspension to be screened to ensure that the apertures in the screen cylinder do not become plugged with pulp and debris. This purpose is achieved by providing the rotor with specific turbulence or rotor elements.
At this stage it should be understood that screening devices whose screen cylinder is rotary, and the means creating turbulence and pressure pulses is stationary, are also known, though more seldom used. The word ‘rotor’ is intended to cover also this kind of turbulence creating means, as they can be said to rotate in relation to the screen cylinder. Also it should be understood that the term ‘screen cylinder’ covers all screening means having openings, i.e. holes or slots, for instance, and having a rotationally symmetric shape. Thus conical or frusto-conical shapes are also covered, and known from prior art.
The pressure screens are most often positioned such that their shaft is in an upright position. However, the pressurization of the fibre suspension makes it possible to position the shaft of a pressure screen in any direction including a horizontal direction. Due to the pressurized feed of the fibre suspension, it may be introduced into a pressure screen to the top, to the bottom or to the centre region thereof.
The pressure screens may also be divided into two groups based on the direction of the accepts flow through the screen cylinder. When the accepts flow radially outwardly, the screen is called an outflow screen, and when the accepts flow radially inwardly, the screen is called an inflow screen.
In accordance with the prior art there are, in principle, two different types of rotors, which are commonly used in the pulp and paper industry and the intention of which, as known, is to maintain the screen surface clean, in other words to prevent blockage of the apertures in the screen cylinder, and maintain sufficient turbulence in the screening zone containing the fresh, i.e. non-screened fiber suspension. These two types of rotors may be called an open rotor and a closed rotor. An example of an open rotor is disclosed in EP-B1-0764736 in which the rotor is arranged inside a stationary screen cylinder. This type of rotor comprises a concentric shaft and a number of turbulence elements in the form of foils extending close to the surface of the screen cylinder. Each foil is supported on the shaft by means of one or more arms extending through the screening zone which contains fresh pulp when the screening apparatus is in operation. The foils may be axial or they may form an angle with the shaft of the rotor and the axis of the screen cylinder. While the foil, or the fibre suspension in relation to the foil, is moving, the trailing surface of the foil subjects the screen surface to a negative pressure pulse for flushing the apertures of the screen cylinder or, rather, for preventing the fibres from accumulating on the screen surface and from blocking the screening openings by means of creating a back flow from the accepts zone to the screening zone.
An example of the other rotor type, i.e. the closed rotor, has been discussed, for instance, in U.S. Pat. No. 3,437,204, in which the rotor is a substantially-cylindrical closed body positioned inside a screen cylinder. The rotor surface in this patent is provided with turbulence elements, i.e. protrusions, which are almost hemispherical in form. In this kind of apparatus, the fresh fibre suspension is fed between the rotor and the screen cylinder, whereby the protrusions of the rotor, the so-called bumps, create turbulence and pressure pulses towards and away from the screen cylinder. In other words, the leading surface of each bump pushes the pulp towards the screen cylinder and the trailing surface of the bump induces a suction pulse that draws the fiber accumulations from the apertures of the screen cylinder. Most often the closed rotor surface is cylindrical. In a broader sense, rotationally-symmetrical rotor surfaces may also be discussed, as there are rotors having a frusto-conical shape or a dome shape. Additionally, there are also rotors not literally having a rotationally symmetrical shape. One such rotor is a so-called S-Rotor, which is formed of two identical cylinder halves attached to each other such that two radially, or substantially radially, arranged surfaces join the half-cylindrical surfaces.
The above mentioned EP-B1-0764746 also teaches that a turbulence element closely resembling a foil may be attached on the surface of a closed rotor. In other words, the turbulence element has a rounded or curved surface, i.e. a convex leading surface between the leading edge of the turbulence element and the peak-line, a line defining the position where the element is at its highest, and a curved trailing surface between the peak-line and the trailing edge of the element. Like the foil of an open rotor, the turbulence element of a closed rotor may extend either continuously from the first end of the rotor to the second end thereof or for a substantial part of the length of the rotor. In a similar manner, the turbulence element may extend axially along the rotor surface or it may form a sharp angle with the axial direction.
Pulp screens are commonly used to remove oversize contaminants, such as plastic specks, fiber bundles or glass fragments from pulp. These contaminants might otherwise reduce the appearance of the paper, tissue, paper board or other products which are made from the pulp. The contaminants might also weaken the paper product or lead to operating problems. In addition, if the contaminants fall out of the partially-formed paper or other paper product, they can foul the equipment used to make the paper products. For any of these various reasons, pulp is screened to remove oversize contaminants from the desirable pulp fibers at an early stage of the pulping and papermaking processes.
While the intent of the pulp screen is to remove contaminants, it must also have a sufficiently high capacity to support the production of the mill and to not limit production. Reduced power consumption is also an objective of improved pulp screening operation.
The two critical components within a pulp screen are the screen cylinder and the screen rotor. The cylinder has small holes or narrow slots through which the fibers pass, but the oversize contaminants do not. The rotor most typically rotates, though there are some pulp screen designs where the rotor is stationary and the screen cylinder rotates. In the typical configuration, where the rotor rotates, the primary objective of the rotor is to ensure that the cylinder does not become permanently plugged with fibers, contaminants and other material and thus unable to process the required flow of pulp. The rotor accomplishes this in two ways. First, the rotor will generate suction pulses which backflush blockages in apertures in the screen cylinder and thus clears the cylinder apertures.
Second, the rotor can also generate three-dimensional turbulence and fluid activity which removes incipient blockages at the aperture entry and applies forces in a multiplicity of directions to help to release any blocked material. Rotors will typically rely mostly or often entirely on the first method, which focuses on blockage removal by simple, mostly radial backflushing pulse. The limited effectiveness of such a single-direction action requires that the rotors rotate at relatively high speeds to provide a strong and frequent backflushing action. Power consumption will tend to be quite high with this approach.
A few rotors have been designed to also provide some three-dimensional activity with the intention of augmenting the main backflushing action, as discussed previously. These other rotors have failed, however, to effectively combine the “activity” and “backflushing” actions. For example, the bump-type (for instance U.S. Pat. No. 3,437,204) and similar (for instance CA-C-1,335,088) type rotor elements generate relatively large-scale three-dimensional activity, but the activity occurs adjacent the pulse generating element and is not effectively coupled with the backflushing action. Likewise another rotor design discussed in CA-C-2,118,410 has foil-type elements, with some ridges attached to the surface of the foil, which provides some small three-dimensional scale activity, but the activity occurs at virtually the same time as the rotor suction pulse and the benefit is lost.
U.S. Pat. No. 5,176,261 discusses a rotor for pressure sorters for sorting fibrous suspensions. The rotor comprises a plurality of cleaning vanes provided for the circulation on the inlet side of a screen cylinder of the pressure sorter, these vanes being designed in sections as return regions and in sections as supply regions; the return regions are designed such that they urge the fibrous suspension portions adjacent the screen inlet side away from the screen cylinder, whereupon these fibrous suspension portions are diverted by the supply regions of the cleaning vanes towards the screen inlet side and fed back to the latter.
U.S. Pat. No. 5,224,603 discusses an apparatus and method for treating fiber suspension. The apparatus is especially applicable for pulp screening in the wood processing industry, particularly for the separation of light particles from fiber suspensions. The apparatus comprises an outer casing with conduits for inlet pulp, accepts, heavier rejects and lighter rejects; a filter cylinder and a rotor, the surface of which is provided with at least one protrusion; and an opening for guiding the light rejects through the surface of the rotor.
DE-A1-10 2011 086 205 discusses a wing for fiber mass sorter. The wing has adjustable fins that are arranged at the end portions of the wing, to prevent ejection of the fiber mass over the ends of the wing. A spring is arranged at guidance portion and a slider suspension is arranged at discharge section. The upper and bottom surfaces are connected with the guidance edge and exit edge of the wing.
Thus the main problems with the prior art rotors are that                they are incapable of creating any three-dimensional activity in connection with the rotor operation (such as with long, foil-type elements on the rotor surface), or        the three-dimensional activity is simultaneous with the radial backflushing, or        the three-dimensional activity created in connection with the rotor elements results in guiding the pulp to be screened to the axial sides of the element and thus weakening the effect of the activity.        
A further problem resulting from the inefficient operation of the prior art rotors is their requirement for higher rotor speed, which means in practice higher power consumption.